Infection is a common source of morbidity in hospitalized patients, many of whom have multiple venous lines, catheters, or other implantable devices in place for extended periods of time. For example, catheter-associated infections represent up to 80% of urinary tract infections in hospitalized patients, yet there is currently no means for detecting whether such an infection exists until the infection has progressed.
Existing practices for management of catheters in a clinical setting rely primarily on statistically-determined guidelines. For example, guidelines state that postoperative urinary catheters should be removed 48 hours post operatively. Additional guidelines exist for treatment of select populations with prophylactic antibiotics. If patient behavior suggests a potential infection, for example, inability to void, fever, etc., a dipstick and urinary analysis is performed. Similarly, if a central line infection is suspected, two sets of blood cultures are typically taken for culture. Urinalysis results can usually be obtained in one to two hours, while urine and blood culture results usually being returned in one to two days. In addition to the disadvantages of the relatively long turnaround time, these tests have a relatively high false positive rate and result in many patients being placed on antibiotics unnecessarily.
Hydrogen peroxide (H2O2) is a toxic byproduct of many physiologic reactions, formed either directly or by enzymes to consume the harmful oxygen free radicals produced during oxidative metabolism. Tissues have evolved sophisticated pathways to control H2O2, using it at low levels for intracellular signaling (<20 μM), at high levels by itself or converted to more harmful oxides for defense (>50 μM), or catalyze it into O2 and water by the enzyme catalase to protect themselves. An imbalance results in elevated levels and is seen in oxidative stress, inflammation, and aging. Hydrogen peroxide is also involved in cancer, diabetes, neurodegeneration, acute respiratory distress (ARDS), and cardiovascular disease. As such, it has potential use for localized detection of a wide variety of biochemical processes in vivo. However, to date, most methods for detecting H2O2 have been confined to in vitro diagnostic use. Prototypes for a few specialized fluorescence, magnetic resonance and genetically encoded probes have been reported, but there is currently no robust injectable probe that can detect H2O2 to localize regions with elevated levels of H2O2 such as areas of inflammation, etc. in humans.
Existing methods include horseradish peroxidase with artificial substrates, which provides high sensitivity in vitro; ferrous oxidation in the presence of xylenol orange; genetically encoded probes such as those incorporating a substrate of SNAP-tag; roGFP or Hyper; MR contrast agents capable of detecting H2O2; enzyme electrodes (such as silica nanowire sensors); (13)C-Benzoylformic acid detection using specialized C-13 hyperpolarized MR sequences; and chemiluminescent nanoparticles.
Currently, detection in collected or voided fluids is compromised by autoxidation when exposed to atmospheric pO2, causing artificial increases in H2O2 levels and decreased accuracy. Fluorescence-based assays, such as Amplex Red or ferrous oxidation of xylenol orange (FOX), are susceptible to contamination by other urine or plasma constituents and are not routinely available in clinical laboratories. Electrochemical and optical-based probes have been developed but are expensive and more difficult to incorporate into routine devices or standard clinical protocols to be practical.
Results have been reported on detection of implant-associated neutrophil responses using a nanoprobe targeting the formyl peptide receptor, however, this is an optical reporter and its clinical use is limited by overlying tissue thickness.
Another technology makes use of increased turbidity detectable in a discharge fluid from an infected catheter tube. The approach also relies on optical imaging and is likely to generate anomalous results in bloody fluids. In other developments, sensors have been designed to monitor the pH of biofilm-producing organisms such as Proteus bacilli, however, these are specific to a single organism.
In view of the foregoing, the need remains for a simple and low cost sensor for monitoring oxidative stress that can be incorporated into existing indwelling devices such as Foley or central venous catheters as a means for detecting H2O2 in real time using conventional clinical instrumentation.